Holger Kraft

A Stochastic Control Approach to Portfolio Problems with Stochastic Interest Rates

We consider investment problems where an investor can invest in a savings account, stocks, and bonds and tries to maximize her utility from terminal wealth. In contrast to the classical Merton problem, we assume a stochastic interest rate. To solve the corresponding control problems it is necessary to prove a verification theorem without the usual Lipschitz assumptions.

Optimal portfolios and Heston's stochastic volatility model: an explicit solution for power utility

Given an investor maximizing utility from terminal wealth with respect to a power utility function, we present a verification result for portfolio problems with stochastic volatility. Applying this result, we solve the portfolio problem for Hestons stochastic volatility model. We find that only under a specific condition on the model parameters does the problem possess a unique solution leading to a partial equilibrium. Finally, it is demonstrated that the results critically hinge upon the specification of the market price of risk. We conclude that, in applications, one has to be very careful when exogenously specifying the form of the market price of risk.Given an investor maximizing utility from terminal wealth with respect to a power utility function, we present a verification result for portfolio problems with stochastic volatility. Applying this result, we solve the portfolio problem for Hestons stochastic volatility model. We find that only under a specific condition on the model parameters does the problem possess a unique solution leading to a partial equilibrium. Finally, it is demonstrated that the results critically hinge upon the specification of the market price of risk. We conclude that, in applications, one has to be very careful when exogenously specifying the form of the market price of risk.

Optimal Housing, Consumption, and Investment Decisions over the Life Cycle

We derive explicit solutions to life-cycle utility maximization problems involving stock and bond investment, perishable consumption, and the rental and ownership of residential real estate. Prices of houses, stocks and bonds, and labor income are correlated. Because of a positive correlation between house prices and labor income, young individuals want little exposure to house price risk and tend to rent their home. Later in life the desired housing investment increases and will eventually reach and exceed the desired consumption, suggesting that the individual should buy his home---and either additional housing units (for renting out) or house price--linked financial assets. In the final years, preferences shift back to home rental. The derived strategies are still useful if housing positions are only reset infrequently. Our results suggest that markets for real estate investment trusts or other house price--linked contracts lead to nonnegligible welfare gains. This paper was accepted by Wei Xiong, finance.

Bankruptcy, Counterparty Risk, and Contagion

This paper provides a unifying framework for the modeling of various types of credit risks such as contagion effects. We argue that Markov chains can efficiently be used to tackle these problems. However, our approach is not limited to pricing problems with contagion. Other applications include the modeling of a more sophisticated default process of a firm. On the theoretical side, we derive pricing formulas for three building blocks that are generalizations of contingent claims studied in Lando (1998). These claims can be thought of as atoms forming the basis for all credit risky payments. Furthermore, we demonstrate that, in general, all contingent claims exposed to credit risk satisfy a system of partial differential equations. This is the key result to calculate prices of credit risky claims explicitly and efficiently.

Optimal Portfolios with Defaultable Securities: A Firm Value Approach

Credit risk is an important issue of current research in finance. While there is a lot of work on modeling credit risk, there is only a few works on continuous-time portfolio optimization with defaultable securities. In this paper we solve investment problems with defaultable bonds and stocks. We assume that credit risk is modeled by a firm value model.

OPTIMAL CONSUMPTION AND INSURANCE: A CONTINUOUS-TIME MARKOV CHAIN APPROACH

Personal financial decision making plays an important role in modern finance. Decision problems about consumption and insurance are in this article modelled in a continuous-time multi-state Markovian framework. The optimal solution is derived and studied. The model, the problem, and its solution are exemplified by two special cases: In one model the individual takes optimal positions against the risk of dying; in another model the individual takes optimal positions against the risk of losing income as a consequence of disability or unemployment.

A Dynamic Programming Approach to Constrained Portfolios

This paper studies constrained portfolio problems that may involve constraints on the probability or the expected size of a shortfall of wealth or consumption. Our first contribution is that we solve the problems by dynamic programming, which is in contrast to the existing literature that applies the martingale method. More precisely, we construct the non-separable value function by formalizing the optimal constrained terminal wealth to be a (conjectured) contingent claim on the optimal non-constrained terminal wealth. This is relevant by itself, but also opens up the opportunity to derive new solutions to constrained problems. As a second contribution, we thus derive new results for non-strict constraints on the shortfall of intermediate wealth and/or consumption.

Elasticity Approach to Portfolio Optimization

Abstract.We study investment problems in a continuous-time setting and conclude that the proper control variables are elasticities to the traded assets or, in the case of stochastic interest rates, (factor) durations. This formulation of a portfolio problem allows us to solve the problems in a kind of two-step procedure: First, by calculating the optimal elasticities and durations we determine the optimal wealth process and then we compute a portfolio process which tracks these elasticities and durations. Our findings are not only interesting in itself, but the approach also proves useful in many varied applications including portfolios with (path-dependent) options. An important application can be the solution of portfolio problems with defaultable bonds modelled by a firm value approach.

Consumption-Portfolio Optimization with Recursive Utility in Incomplete Markets

In an incomplete market, we study the optimal consumption-portfolio decision of an investor with recursive preferences of Epstein–Zin type. Applying a classical dynamic programming approach, we formulate the associated Hamilton–Jacobi–Bellman equation and provide a suitable verification theorem. The proof of this verification theorem is complicated by the fact that the Epstein–Zin aggregator is non-Lipschitz, so standard verification results (e.g. in Duffie and Epstein, Econometrica 60, 393–394, 1992) are not applicable. We provide new explicit solutions to the Bellman equation with Epstein–Zin preferences in an incomplete market for non-unit elasticity of intertemporal substitution (EIS) and apply our verification result to prove that they solve the consumption-investment problem. We also compare our exact solutions to the Campbell–Shiller approximation and assess its accuracy.

Asset allocation and liquidity breakdowns: what if your broker does not answer the phone?

This paper analyzes the portfolio decision of an investor facing the threat of illiquidity. In a continuous-time setting, the efficiency loss due to illiquidity is addressed and quantified. For a logarithmic investor, we solve the portfolio problem explicitly. We show that the efficiency loss for a logarithmic investor with 30 years until the investment horizon is a significant 22.7% of current wealth if the illiquidity part of the model is calibrated to the Japanese data of the aftermath of WWII. For general utility functions, an explicit solution does not seem to be available. However, under a mild growth condition on the utility function, we show that the value function of a model in which only finitely many liquidity breakdowns can occur converges uniformly to the value function of a model with infinitely many breakdowns if the number of possible breakdowns goes to infinity. Furthermore, we show how the optimal security demands of the model with finitely many breakdowns can be used to approximate the solution of the model with infinitely many breakdowns. These results are illustrated for an investor with a power utility function.